The invention relates to a device for locking a freewheeling unit. Further in vehicles with a drive unit to transmit torque from a first driving axle to a second driving axle, the system includes at least one freewheeling unit and a slip coupling connecting the axles. A freewheeling unit includes a first drive element, connected to the drive unit and a second drive element, connected to the rear axle. The freewheeling unit in its unlocked operating condition, enables the wheels of the front axle to overtake the wheels of the rear axle.
Four wheel drive vehicles frequently comprise a directly driven axle, normally the front axle, and a non-directly driven rear axle. If a speed differential occurs between the two axles, the non-directly driven axle is torque-loaded via a slip coupling. Slip couplings suitable for this purpose are viscous couplings, for example, which in this arrangement are also referred to as viscous transmissions.
Other suitable couplings are based on a pumping effect. Furthermore, there are prior art four wheel drives which, in an axle differential in the non-directly driven axle, is arranged with two slip couplings in a drivable housing. The couplings in this arrangement are referred to as torque splitters with the respective second coupling halves being connected to one of the axle shafts, respectively. Also in this arrangement, a torque is transmitted to the non-directly driven axle if there is a speed differential between the two axles, with at the same time the slip couplings replacing the differential.
With the drive systems mentioned here and with a direct connection of the axles via the coupling, torque would also be transmitted from the non-directly driven axle to the directly driven axle. Generally this occurs during braking when there is a speed differential between a locking front axle and the rear axle. Even when using an anti-lock braking system, there occurs a speed differential between the front and the rear axle due to driving stability and that the circumferential slip of the rear wheels with respect to the ground is kept smaller than the circumferential slip of the front wheels.
DE-OS No. 37 08 193 illustrates integration of a freewheeling device with a freewheeling lock on the propeller shaft. The device, in its unlocked condition, permits a higher speed of the rear axle wheels as compared to the front axle wheels; i.e. it permits overtaking of the rear wheels with respect to the front wheels. During normal driving conditions, the freewheeling unit is locked, with unlocking being effected by actuation of the vehicle brakes. Thus, during the braking operation, the rear wheels are allowed to overtake. The disadvantage of this system is that control of the freewheeling unit is only possible during actuation of the brake pedal.
U.S. Pat. No. 4,889,353 proposes a drive unit which transmits torque between a rotating input and output shaft. The patent shows a coupling which ensures the transmission of torque from the input shaft to the output shaft and vice versa within a low speed range and which, in a higher speed range, permits torque only to be transmitted from the input shaft to the output shaft. However, this design involves a lockable drive unit and is complicated and expensive.
DE-AS No. 11 37 636 illustrates a semi-automatic motor vehicle coupling consisting of a separating coupling and a centrifugal force coupling including a freewheeling unit arranged at the hub extension of the flywheel. To improve the support of the two couplings inside each other and to improve the arrangement of the freewheeling lock, it is proposed to separate the coupling, via a roller bearing, at the same time the freewheeling lock is supported on the outer casing of the hub extension of the flywheel of the centrifugal force coupling. The flywheel is centered on and connected to the crankshaft with the centrifugal weights supported relative to the coupling housing. This reference, however, does not teach locking of the centrifugal weights.
It is the object of the present invention to provide a freewheeling unit which is effective in one direction of rotation and which, as a function of the number of revolutions driving it, enables automatic locking in order to enable both axles to be driven during reversing (driving backwards).
In accordance with the invention, the objective is achieved by a first drive element with at least one circumferentially distributed aperture. A locking element, which is radially displaceable for the purpose of establishing a non-rotating connection between the first and second drive element, is positioned in the aperture. The locking element is partially transferred into at least one recess of the second drive element which corresponds to the aperture to establish the non-rotating connection. An axially movable spring-loaded control element, with a locking face to displace the locking elements, is arranged coaxially relative to the drive elements. The control element, which is axially movable by radial displacement of centrifugal masses when a predetermined nominal speed is exceeded, releases the locking elements, and thus the non-rotating connection between the drive elements. When the freewheeling unit is in the unlocked operating condition, torque is transmitted by the freewheeling unit only in the main driving direction of rotation.
When the freewheeling unit is in the locked operating condition, the locking elements in the apertures and recesses of the first and second drive element transmit torque when driving in the backwards direction. The locking elements transmit negative torque under pushing vehicle conditions, when driving in the forward direction, to eliminate the function of the clamping roller freewheeling unit. Such a freewheeling unit may be used in the propeller shaft of a motor vehicle. For example, at low vehicle speeds, the locking elements transmit torque when driving in the backwards direction and they transmit torque under pushing vehicle condition, when driving in the forward direction. At higher vehicle speeds, torque transmission when driving in the forward direction is taken up by the freewheeling unit to enable the rear axle to overtake during braking or under pushing vehicle conditions.
In one embodiment of the invention the control element is an axially adjustable shaft journal. The journal is coaxially guided in the first drive element and at one end includes a locking face. In a locking position, the locking face holds the locking elements radially outside, through the apertures of the first drive element, between the locking face and the corresponding recesses of the second drive element. The locking face, in a releasing position, enables the locking elements to enter an axially close hollow space, which as a result, releases the non-rotating connection between the first and second drive element.
According to a further embodiment of the invention, the first drive element and/or the second drive element is a shaft, a flange or a hollow shaft.
An advantage of the invention is that it achieves a compact design with the drive elements being supported coaxially inside each other. Also, the invention accommodates the freewheeling unit so that the freewheeling unit may be used even if only a small amount of installation space is available.
According to a further feature of the invention, the first drive element is connected to an outer ring of the freewheeling unit. The second drive element is provided with a joint yoke or a connecting flange. This achieves a compact design which is already provided with further drive elements.
According to a further embodiment of the invention, the control element is an axially adjustable sleeve with an internally positioned locking face. The control element is coaxially guided in the first drive element and, in a locking position, holds the locking elements radially inside or radially outside between the locking face and the recesses of the second drive element. In a releasing position, the locking face enables the locking elements to leave the recesses, which, as a result, releases the non-rotating connection between the first and second drive elements. The advantage of this design is that when the non-rotating connection is released, the locking elements cannot enter the recesses of the second drive element so that no noise can develop when a speed differential occurs.
According to a further embodiment of the invention, a second end of the control element is connected to a dish-shaped disc. The disc together with the end wall of the flange shaft forms a hollow space which is tapered towards the outside and which contains centrifugal masses. The centrifugal masses axially move the control element against the force of a spring when the predetermined speed is reached, which, as a result, releases the non-rotating connection.
The force of a spring holds the control element in the locking position. Only through the effect of the centrifugal forces, which moves the control element in the axial direction, when the speed of the drive elements exceeds a nominal speed is the releasing position achieved. When the vehicle reverses slowly or drives forward slowly, the returning force of the spring is greater than the axial force component generated by the centrifugal masses so that the control element is held in the locking position.
According to yet a further embodiment of the invention, it is proposed that opposite the aperture of the first drive element there should be provided circumferentially distributed recesses. Centrifugal masses are radially guided in the recess. In a region surrounding the centrifugal masses, the control element axially extends so as to be curved radially outwardly. The control element is axially movable against the force of a spring which, as a result, releases the non-rotating connection when the predetermined nominal speed is achieved.
Again, the locking elements are held in a locking position by a control element. The locking elements are always released when the nominal speed of the drive elements is exceeded and when, due to the effect of the centrifugal masses, the switching sleeve is moved axially. The centrifugal masses are guided in apertures of the first drive element and are pushed into the recesses of the second drive element by the locking face of the switching element, thereby allowing the transmission of torque via the locking elements. In a further embodiment, the locking elements are simultaneously designed as centrifugal masses. At the same time, the locking elements and/or the centrifugal masses may be designed as balls.
From the following detailed description taken in conjunction with the accompanying drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.